4.1 Introduction

Over the years the semiconductor industry has endeavoured to provide circuit designers with a cheap, high quality, versatile analogue building-block and many amplifier designs have been manufactured with the sole aim of producing a controlled voltage output from a voltage input. The most popular of these networks is the high gain voltage-mode operational amplifier (VOA). It has, thus, become customary for electrical engineers to think of analogue signal processing in terms of voltage variables and this tendency has resulted in numerous voltage signal processing circuits. For example, the operational amplifier is conveniently configured into controlled voltage source amplifiers, such as the voltage controlled voltage source (VCVS) and current controlled voltage source (CCVS), but not so easily configured into controlled current source amplifiers such as the voltage controlled current source (VCCS) and current controlled current source (CCCS).

There is often a demand in analogue signal processing for amplifier circuits that possess well-defined current signal processing properties. Furthermore, current amplifier based circuits can offer certain high performance properties, such as speed, bandwidth and accuracy, which make them preferable to voltage amplifier designs. In view of this there is an obvious need for high quality current amplifier based analogue networks, which can provide comparable, if not superior, performance to traditional voltage amplifier designs. This potential has been recognised by many circuit designers, and a major area of research over the past two decades has been the design and application of current-mode techniques.

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